Process for preparing silicon carbide powder from vaporized polysiloxanes

Abstract: Disclosed is a process for the formation of silicon carbide powder in which vaporized polysiloxanes are reacted and pyrolyzed in a single heating step to form silicon carbide powder. The process is simple, inexpensive and yields powder having desirable characteristics.

Description: The present invention relates to a process for preparing silicon carbide powder from vaporized polysiloxanes. This process is characterized in that our polysiloxane is reacted and pyrolyzed to form the SiC powder in a single step. The powder produced by this process, moreover, is desirable in that it is non-agglomerated and can be densified to form silicon carbide bodies. Silicon carbide ceramics have utility in numerous applications because of their desirable properties and characteristics. The production of such ceramics, however, requires silicon carbide powder which is inexpensive, pure and in a form which can be easily sintered to a ceramic. Silicon carbide powder is produced by a number of known techniques using a variety of silicon containing compounds. For example, SiC can be formed by vapor phase pyrolysis of alkylated silanes (Canadian Patent 592,456); mixtures of silicon halides and hydrocarbon gases (US Patents 3,346,338 and 4,295,890); and polysilanes (US Patents 4,676,966 and 4,571,331). Silicon carbide can also be formed by the reaction of silica and carbon or by the pyrolysis of a variety of non-vapor phase organosilicon polymers, such as polysiloxanes (US Patent 4,840,781); polycarbosilanes (US Patent 4,465,647); and polysilanes (US Patent 4,534,948). However, the use of vaporized polysiloxanes for forming the SiC powder is not taught in these references. US Patent 4,613,490 teaches a process for forming SiC powder in which a polysiloxane undergoes a vapor phase reaction to form a SiCO product at a temperature less than 1600°C. that is then heated (calcined) to form the resultant powder. This process is time consuming and expensive because it requires a second heat (calcining) step to form the powder. Additionally, the powder prepared by this 2-step process is often highly agglomerated and is difficult to densify. The present invention is a process as set forth in claim 1. The present invention is based on our unexpected finding that silicon carbide powder can be formed by a single-step vapor-phase reaction and pyrolysis of a polysiloxane. This is particularly surprising since the prior art taught that the SiC derived from a polysiloxane required an additional pyrolysis step. Avoiding this second step provides substantial savings in both time and capital in our process. In addition, the SiC powder produced by our process has characteristics such as desirable size distributions (without grinding), morphology, high purity and a good densification profile. This was also unexpected since conventional processes produce powders which are highly agglomerated which require grinding or milling with media (that can be a source of impurities) to produce a powder which satisfactorily densifies. The polysiloxanes used in the process of this invention comprise any which are either in their vapor phase at room temperature or which can be vaporized by heating. The vaporization temperature, however, should be low enough that the polysiloxane is vaporized before it begins to decompose. We postulate that decomposition of the polysiloxane in the vapor state provides the finely divided powders which we produce. The useful polysiloxanes include, for example, cyclic polysiloxanes of the structure [R 2 SiO] n wherein each R independently comprises a hydrogen atom or a hydrocarbon of 1-6 carbon atoms and n is an integer of 3-8. Examples of R groups include alkyls such as methyl, ethyl, propyl and butyl; alkenyls such as vinyl, allyl and hexenyl; alkoxy such as methoxy and ethoxy; or cyclic groups such as cyclopentyl, cyclohexyl or phenyl. Also contemplated are cyclic polysiloxane copolymers. Other examples of suitable polysiloxanes included linear polysiloxanes of the structure R 3 Si-O-[R 2 SiO] n -SiR 3 wherein each R independently comprises a hydrogen atom or a hydrocarbon of 1-6 carbon atoms and n is an integer of 0-8. Examples of R groups include alkyls such as methyl, ethyl, propyl and butyl; alkenyls such as vinyl, allyl and hexenyl; alkoxy such as methoxy and ethoxy; or cyclic groups such as cyclopentyl, cyclohexyl or phenyl. Again, copolymers are also contemplated. Additionally, it is contemplated that any polysiloxane which will evolve volatile polysiloxanes upon heating can be used herein. For instance, a broad molecular weight polymer can be used even though only the low molecular weight species will be volatilized. Similarly, any polymer which will decompose upon heating to evolve volatile species can also be used. Finally, it is also contemplated that mixtures of any of the above polysiloxanes can be used in the process of the present invention. Naturally, it is anticipated that the stoichiometry of the SiC powder (i.e., the Si/C ratio) produced by our process will vary depending on the polymer and its substituents. For instance, polymers with unsaturated alkyls or phenyl groups will likely contain excess carbon in the SiC powder. Such variability, however, is valuable in that it allows one to engineer SiC powders with any desired stoichiometry. As noted above, the polysiloxane must be in the vapor phase to be useful in the present invention. If the polysiloxane is not in the vapor phase at room temperature, it must first be vaporized. This is generally accomplished by heating the polysiloxane to a temperature above its boiling point. Typically, the boiling point of the polysiloxane should be 800°C. or below, preferably below 600°C. and most preferably below 400°C. for use in our invention. The use of alternative techniques which are known in the art for producing vapors, however, are also contemplated herein. For instance, the use of a vacuum or of a gas to decrease the partial pressure of the polysiloxane vapor below the equilibrium vapor pressure of the polysiloxane liquid is contemplated herein. The vaporized polysiloxane is then fed into a reaction chamber. The polysiloxane vapor may be fed into the reaction chamber by itself (i.e., neat) or it may be diluted in an inert gas such as argon or helium. When diluted, the concentration of polysiloxane vapor is generally not critical and can vary over a wide range (eg., 0.01 volume percent and above). Whether dilute or neat, enough polysiloxane vapor is fed into the reaction chamber to produce the desired product. The vapor may be introduced as a continuous flow through the reaction chamber or the vapor may be introduced and the flow stopped to increase residence time. Obviously, the amount of vapor introduced into the reactor will be determined by factors such as the size of the reactor, the pyrolysis temperature, the desired yield and the like. We have also discovered that the amount and rate of the vapor affects powder characteristics such as yield, morphology, size and chemistry. As such, we contemplate that controlling these variables will allow one to produce a specific powder for a given utility. Generally, the pressure in the reaction chamber is not critical. Pressures in the range of between 0.001 up to several atmospheres are generally useful herein. The reaction chamber used in the invention is not critical and any which can be heated to the temperature necessary for reaction and pyrolysis can be used herein. For instance, reactors such as a tube furnace or those described in the examples can be used herein. Once the polysiloxane vapor is in the reactor, the pyrolysis of the vapor to form SiC containing powder and the calcination of this powder to form the desired product can be initiated by a single heating step. Generally, initiation of this reaction is spontaneous in the presence of the elevated temperature adopted herein. In simplest form, the invention can comprise feeding the polysiloxane vapor into a heated reaction chamber wherein the polysiloxane will react and be heated to form the desired product. It is contemplated, however, that alternative approaches such as introducing the vapor into a reaction chamber followed by heating the chamber to the desired temperature or using a plasma to initiate the reaction are also feasible for the present invention. The temperature within the reaction chamber should be sufficient to insure complete conversion of the polysiloxane to silicon carbide. If inadequate temperatures are used, sufficient oxygen is not lost and silicon oxycarbide powders can be formed. Generally, the temperatures should be above 1600°C., preferably at least 1800°C. and more preferably at least 2000°C. The residence time of the vapor within the reaction chamber should be sufficient to insure complete conversion of the polysiloxane to silicon carbide. Generally, times in the range of a fraction of a second up to several hours can be used, depending on the reaction chamber. Usually, shorter residence times are preferred (eg., 0.01 seconds up to 30 minutes). Herein, we have shown that factors such as temperature and residence time at a temperature will affect powder characteristics such as morphology and particle size. As such, these factors can be controlled to engineer a desired powder. After the polysiloxane is converted to the desired silicon carbide powder in the reaction chamber, the powder is merely collected. The method of collection is not critical and nearly any practical means can be used. The resultant powders are generally finely divided and can be produced in a wide variety of sizes and morphologies. For instance, particle sizes in the range of 0.001 to 10 micrometers can be produced (having either narrow or broad size distributions) and morphologies of cubic (beta), non-cubic (alpha) and mixtures thereof can be produced. Our powders can also have nearly any stoichiometry desired from stoichiometric SiC to excess silicon or carbon. Moreover, small amounts of oxygen (eg., less than 5%) may also be present. Finally, the powders are generally pure (i.e., low levels of impurities) because the only reactant in our process is the polysiloxane which is provided in a pure form. Such powders have distinct advantages over powders produced by alternative, 2-step heating processes. For instance, powders produced in conventional silica-carbon processes have high levels of contaminants which require extensive cleaning. Similarly, powders produced by alternative processes are often agglomerated such that grinding and/or milling are frequently necessary prior to their use. Finally, our silicon carbide powders have a good particle size distribution which provides high green densities. The following non-limiting examples are provided so that one skilled in the art may better understand the invention. Examples 1-4 (Comparative) Octamethylcyclotetrasiloxane ([(CH 3 ) 2 Si-O] 4 ) was vaporized in a bath at 226°C. The resultant vapor was fed at 4 cm 3 /min into a tube furnace accompanied by an argon flow of 3.22 L/min. The tube furnace was maintained at a reaction temperature of 1400°C. and the residence time of the vapor in the furnace was about 2 minutes. The resultant powder was dark grey and had a composition comprising 55.61 wt% Si, 12.81 wt% O and 31.58 wt% C (by difference). The powder had a density of 2.79 g/cm 3 and a crystallite size of 7.7 nm (77 Angstroms). X-ray diffraction analysis of the powder showed 94 wt% beta SiC and 6 wt% C in the crystalline phase. Samples of the above powder were loosely packed in a graphite crucible and heated in flowing argon (calcined) at 1500°C. (Example 1), 1600°C. (Example 2), 1800°C. (Example 3) and 1900°C. (Example 4). The results are presented in Table 1. Characteristics of Calcined Powder Characteristics Ex 1 1500°C Ex 2 1600°C Ex 3 1800°C Ex 4 1900°C Color Yellow-white Light grey Light green Dark green Composition (wt %) Si 62.87 68.23 66.42 69.01 O 2.22 0.43 0.14 0.11 C (difference) 34.91 31.34 33.44 30.88 Calculated excess Carbon -- 2.2 5.3 1.3 X-ray Diffraction (wt %) beta SiC 70 96 98 95 alpha SiC 30 4 -- -- Si -- -- -- -- SiO -- -- -- C -- -- 2 5 Particles Size (micrometers) 0.05-0.2 0.2-0.5 0.5-3 2-5 Some of the above powder was ball milled. Samples of the milled and unmilled powders were mixed into powder formulations for sintering. The mixtures comprised 87.4 wt% SiC powder, 0.33 wt% B 4 C, 11.93 wt% polysiloxane binder of the structure (PhSiO 1.5 ) 0.27 (MeSiO 1.5 ) 0.18 -(Me 2 SiO) 0.18 (MeViSiO) 0.25 (Ph 2 SiO) 0.12 (as prepared in Example 11 of US Patent 4,888,376), 0.24 wt% Lupersol™ (2,5-bis(t-butylperoxy)-2,3-dimethylhexane) and 0.55 wt% of a dispersant comprising polyolefin aminoester salt and diethylethanolamine. The mixtures were prepared by sonication of the above materials in a mixture of ethanol and toluene followed by stripping away the solvents. The resulting dry mass was ground and sieved at 0.15 mm (100 mesh). The powders were then compacted to green bodies (pellets) at 20.7 GPa (30 Ksi). The results are given in Table 2. Compaction Behavior of SiC Powders Ex No. Green Density (g/cm 3 ) (No Ball Milling) Green Density (Ball Milled) 1 could not be formed --- 2 1.49 1.89 3 1.91 --- 4 2.13 --- The above green bodies were then sintered at 2070°C. for 1 hour under argon. Table 3 reports the results. Characteristics of Sintered Bodies Ex No. Milling Sintered Density (g/cm3) XRD Phase Content (%) Beta Alpha 1 no 2.15 37 63 2 yes 2.5 11 89 3 no 2.34 0 100 4 no 2.19 39 61 Examples 5-7 Octamethylcyclotetrasiloxane ([(CH 3 ) 2 Si-O] 4 ) was vaporized and fed into the induction furnace of the Figure. The run conditions are set forth in Table 4. The characteristics of the resultant powder are set forth in Table 5. Run Conditions EX No Siloxane Feed Rate (ml/min) Argon Flow (ml/min) Argon Blanket (ml/min) Nozzle Temp (°C) Max. Furnace Temperature (°C) Column Gauge Pressure (kPa) 5 4 >10 >10 500-700 2000 0-4.8 6 4 4 4 500-600 2000 0-2.1 7 4 4 4 500-600 1800 0-2.1 Characteristics of Powder Characteristics Example 5 (comparative) Example 6 Example 7 Crystal Structure beta beta beta Morphology highly agglomerated not agglomerated not agglomerated Particle Size (micrometer) 0.1-0.5 0.3-2 0.1-1 Sp. Surface Area (m 2 /g) 11.51 5.46 8.92 Oxygen Content (%) 3.09 0.68-0.7 2.58 Flow Charact. of Raw Powder Fill Density 0.156 0.441 0.274 Tap Density 0.205 0.352 0.352 Green Density (g/cm 3 ) 1.82 2.35 2.20 Fired Density* (g/cm 3 ) 2070°C no densification 2.95 2.98 2100°C 2.97 2.98 XRD Phase Cont. 2070°C. beta not measured 56.6% 53.5% alpha 43.0% 46% carbon 0.4% 0.5% 2100°C. beta 25% 47.6% alpha 74.4% 52% carbon 0.6% 0.4%

Claims: A single-step heating process for preparing silicon carbide powder comprising: introducing a vaporized polysiloxane at a temperature of from 500 to 600°C into a reaction chamber; heating the polysiloxane vapor in the reaction chamber to a temperature of 1600°C. or above for a time of 0.01 second to 30 minutes to form silicon carbide powder; and collecting the silicon carbide powder. The method of claim 1 wherein the polysiloxane comprises (a) cyclic polysiloxanes of the structure [R 2 SiO] n wherein each R independently comprises a hydrogen atom or a hydrocarbon of 1-6 carbon atoms and n is an integer of 3-8; or (b) a linear polysiloxane of the structure: R 3 Si-O-[R 2 SiO] n -SiR 3 wherein each R independently comprises a hydrogen atom or a hydrocarbon of 1-6 carbon atoms and n is an integer of 0-8. The method of claim 2 wherein the polysiloxane comprises cyclic polysiloxanes of the structure [(CH 3)2 SiO] n where n = 3-8. The method of claim 1 wherein the vaporized polysiloxane is in an inert gas. The method of claim 1 wherein the pressure in the reaction chamber is between 0.1 and 202.6 kPa (0.001 and 2 atmospheres).

Inventors: ATWELL WILLIAM HENRY, BARTOS DONALD MICHAEL, HARDER PATRICK JAMES, LIPOWITZ JONATHAN, SAHA CHANDAN KUMAR, ATWELL, WILLIAM HENRY, BARTOS, DONALD MICHAEL, HARDER, PATRICK JAMES, LIPOWITZ, JONATHAN, SAHA, CHANDAN KUMAR
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C01B31
IPC codes: C04B35, C01B31